10 E. R. HOSKINS AND M. M. HOSKINS 



1917. The frogs which formed from the larger larvae were 

 larger than those formed from smaller larvae. In Rana sylvatica 

 there is thus a great variation in the size of the newly formed 

 frogs. As shown in table 2, the body length of these normal 

 frogs of the same stage of development varies more than 25 per 

 cent and their volume nearly 100 per cent. 



The mortality in 1917 was very great, but was about the 

 same in both the thyroidless and control animals. However, 

 42 of the former and 20 of the latter reached the period of normal 

 metamorphosis. Of these (table 1) the controls all metamor- 

 phosed normally. Of the operated animals, 12 metamorphosed 

 and 30 failed to do so. By autopsy and sectioning it was later 

 ascertained that every one of the 30 was devoid of thyroid, and 

 in every one of the 12 the gland had regenerated after the oper- 

 ation. In the 1918 work, out of 47 thyroidless animals which 

 reached the period of normal metamorphosis, 44 failed to meta- 

 morphose and in only three experimental larvae did the thyroid 

 regenerate permitting metamorphosis. In the three seasons a 

 total of 91 experimental larvae were reared to or beyond the 

 time of normal metamorphosis, and of these 75 remained in the 

 larval condition. In addition to these, many which were killed 

 in the early stages for purpose of study would doubtless have 

 survived long enough to be added to the 75. From these experi- 

 ments and those of Allen ('18) we are justified in the conclusion 

 that removal of the thyroid from young frog larvae will delay 

 if not entirely prevent metamorphosis under ordinary laboratory 

 conditions. Most of the 75 thyroidless larvae just referred to 

 were killed after it became evident that they would not develop 

 into frogs, but a few were kept alive. Of these, six survived 

 the winter and the second season of normal metamorphosis with- 

 out becoming frogs. It is more or less common knowledge that 

 if Amblystoma larvae kept in aquaria are not properly fed, 

 they may fail to undergo metamorphosis their first season, but 

 do metamorphose the second season. The same thing is true 

 also in nature. Dr. T. G. Lee, of the University of Minnesota, 

 has told us that he has collected in the early spring Amblystoma 

 larvae which should have metamorphosed the previous season. 



